Xfrmr Primary/Sec OCPD Help

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Jerramundi

Jerramundi

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Licensed Residential Electrician
Looking at the option of installing a 240V/120V 3000VA Step-Down Isolating Transformer for a project. Need some clarification on the Primary/Secondary protection rules.

I chose 3000VA because the sign needs to be fed with a 20A circuit per the 600's, although the 600's doesn't specify what the voltage rating of that circuit must be.

In my mind, I start with 120V * 20A = 2400VA. Since 2.5kVA xfrmr seems to be rather uncommon, I went with 3kVA because (1) it's more common and (2) I would not be loading the xfrmr to the near maximum capacity per its' nameplate rating, ideally increasing the longevity of the unit.

So, say I want my Secondary to be limited to 20A @ 120V or 2400VA...

If I do *Primary Protection ONLY*...

For the Primary OCPD, that's 2400VA/240V = 10A. That's greater than 9A, so per Table 450.3(B), Max OCPD = 10A x 1.25 = 12.5A (next size up yields 15A OCPD).

However, and I know this is rather theoretical and OCPD tolerances are different, but say the circuit, for whatever reason, pulls 14.99A, not tripping that OCPD.

That's 14.99A x 240V = ~3600VA, which is greater than the transformer VA rating. How is this allowed if theoretically the circuit could pull more power than the xfrmr is rated for? Am I doing something wrong? Can most transformers handle MORE than their nameplate rating??

If I do BOTH *Primary AND Secondary Protection*...

For the Secondary OCPD, that's 2400VA/120V = 20A. That's greater than 9A, so per Table 450.3(B), Max OCPD = 20A x 1.25 = 25A (25A is considered a standard size per 240.6, even though I kind of chuckle at this notion).

Again, utilizing the same logic as above, that's 25A x 120V = 3000VA (maxed, but safe). This would also prevent the primary circuit from exceeding 12.5A, which eliminates the Primary protection problem of exceeding the xfrmr VA rating.

It seems to me, the best solution is to do BOTH Primary and Secondary protection....
 
The values in Table 450.3(B) are based on the Transformer-Rated Current.

For a 3kVA, 240-120V transformer that would be 12.5A rated primary current, 25A rated secondary current.

For primary only protection, the Primary OCPD could be 20A

For primary and secondary protection, the Primary OCPD could be 30A and the secondary OCPD could be 35A.
 
david luchini said:
The values in Table 450.3(B) are based on the Transformer-Rated Current.
Click to expand...
Understood. These are the MAXIMUM allowable OCPD ratings, correct?

david luchini said:
...For primary and secondary protection, the Primary OCPD could be 30A and the secondary OCPD could be 35A.
Click to expand...
For Primary and Secondary, in regards to the Primary OCPD, don't you get 12.5A x 250% = 31.25 (upsized to 35A?) So I guess, yes, it could be 30A, but no more than 35A.
 
david luchini said:
The values in Table 450.3(B) are based on the Transformer-Rated Current.

For a 3kVA, 240-120V transformer that would be 12.5A rated primary current, 25A rated secondary current.

For primary only protection, the Primary OCPD could be 20A

For primary and secondary protection, the Primary OCPD could be 30A and the secondary OCPD could be 35A.
Click to expand...
My question is more so in regards to why this is allowed.

Looking at the example you gave w/ a 30A Primary OCPD and 35A Secondary OCPD (maximum allowable OCPD's)
30A on the Primary @ 240V would pull 7200W, which is twice the 3kVA rating of the xfrmr. How is this allowable?
 
Jerramundi said:
Understood. These are the MAXIMUM allowable OCPD ratings, correct?
Click to expand...

Yes, T450.3(B) gives the Maximum allowable OCPD ratings based on transformer-rated current.


Jerramundi said:
For Primary and Secondary, in regards to the Primary OCPD, don't get 12.5A x 250% = 31.25 (upsized to 35A?) So I guess, yes, it could be 30A, but no more than 35A.
Click to expand...

No more than 30A...can't be upsized to 35A.
 
Jerramundi said:
Looking at the example you gave w/ a 30A Primary OCPD and 35A Secondary OCPD (maximum allowable OCPD's)
30A on the Primary @ 240V would pull 7200W, which is twice the 3kVA rating of the xfrmr. How is this allowable?
Click to expand...

This example would allow you to load the transformer to 40% beyond its rating, not twice its rating. Doesn't mean you have to load it that heavily.
 
david luchini said:
This example would allow you to load the transformer to 40% beyond its rating, not twice its rating. Doesn't mean you have to load it that heavily.
Click to expand...
Where do you get 40% from? Is it because 35A Secondary OCPD x 120V = 4200VA.. 4200VA-3000VA = 1200VA.. 1200VA/3000VA = 40%?
Basically the secondary OCPD limits the maximum of the primary.
 
Okay, all caught up there... but as far as loading it beyond the rating, why?

What am I missing about transformer construction that allows you to go even 1VA beyond the rating?
 
Jerramundi said:
Okay, all caught up there... but as far as loading it beyond the rating, why?

What am I missing about transformer construction that allows you to go even 1VA beyond the rating?
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A transformer has a large thermal mass that allows for short term overloads as long as it is allowed to cool in between. Small percentage overloads are not catastrophic but result in a reduced operating life.
And like any other load and OCPD combination, with the specific exception of motor overloads, we rarely size the OCPD to the exact working rating of the loads it serves. It is there primarily to handle short circuits and ground faults, while small percentage overloads will not be prevented, potentially reducing the life of the load after it has begun to malfunction.

Another way of looking at it is that small transformer overloads are a system design rather than safety issue.
 
GoldDigger said:
A transformer has a large thermal mass that allows for short term overloads as long as it is allowed to cool in between. Small percentage overloads are not catastrophic but result in a reduced operating life.
And like any other load and OCPD combination, with the specific exception of motor overloads, we rarely size the OCPD to the exact working rating of the loads it serves. It is there primarily to handle short circuits and ground faults, while small percentage overloads will not be prevented, potentially reducing the life of the load after it has begun to malfunction.

Another way of looking at it is that small transformer overloads are a system design rather than safety issue.
Click to expand...
Oh, so we're taking a page out of Bill Gate's playbook to allow for that "anti-virus software money?" Haha

No I hear you. I was just reading an article about the differences between base ratings, normHkVA, and emergHkVA. You're right. It is a part of the design.
 
As far as clearing a ground fault is concerned... are there benefits to including the secondary OCPD too instead of just the primary?
 
Jerramundi said:
True. But I'm still unclear as to why you're allowed to load the xfrmr even 1VA beyond its' rating...
Click to expand...

The NEC is absolutely silent on how to size transformers (and motors). This is a design consideration that needs to be discussed with your equipment manufacturer.
NEC 450 is about how to protect a transformer based on the size you selected.

Sizing is about heat build up, as mentioned by golddigger, and involves how much heat gets generated in the core of the transformer, how long that heat lasts and how long the transformer gets to cool down before the heat builds up again. Overloaded transformers have reduced life.
 
Something about a primary only OCPD, especially on an SDS, just makes me feel uncomfortable, lol.

I understand that with the proper grounding and bonding requirements, the secondary side of the SDS is basically operating in the same way a service would, but still.
 
Jerramundi said:
As far as clearing a ground fault is concerned... are there benefits to including the secondary OCPD too instead of just the primary?
Click to expand...

Ground faults typically do not transfer across a delta-wye transformer configuration, so secondary side protection is usually based on load requirement/benefits. A common transformer failure is a winding shorting to the grounded core, this is why the primary needs protection.
 
jim dungar said:
...secondary side protection is usually based on load requirement/benefits....
Click to expand...
By this I assume you mean protecting the secondary conductors as well as the ability to divide the secondary into multiple different branch circuits?
 
jim dungar said:
Ground faults typically do not transfer across a delta-wye transformer configuration, so secondary side protection is usually based on load requirement/benefits. A common transformer failure is a winding shorting to the grounded core, this is why the primary needs protection.
Click to expand...
Understood. But if the secondary winding shorted to the grounded core, or anything on the secondary side for that matter, despite obviously demanding more from the primary and thus tripping the primary OCPD, would a secondary OCPD not theoretically "react" more quickly?
 
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